Direct conversion receivers immediately down convert a received radio signal to a baseband signal thereby completely eliminating the need for an intermediate frequency (IF) stage. Direct conversion receivers are often referred to as Zero IF receivers. Such receivers suffer from the formation of a very large, unwanted DC component interfering with the baseband signal, formed largely by leakage from the local oscillator being received at the antenna along with the desired RF signal. The DC component may be further impacted by temperature, baseband circuit characteristics, IC processes, and/or transients to name a few. The DC component also affects offsets of the amplifiers and mixers in the receiver.
The design of a direct conversion receiver (Zero IF) faces two main challenges in analog FM Applications. The first challenge pertains to the DC offset error present in the baseband signal. The second challenge pertains to baseband imbalance caused by gain and phase imbalance in the mixers.
Past approaches for addressing the DC offset error and gain/phase imbalance have included applying a high pass filter after the analog-to-digital converter (ADC). The high pass filter approach however, is not acceptable for analog FM radio applications, especially when the radio is operating in a subaudible signalling mode, such as FM digital private line (DPL) mode, Tone private line (PL) mode, where a low level digital signal is transmitted rather than a continuous audio tone, or other mode where low frequency sub-audible signals are present. The subaudible signals can be further distorted by the high pass filter and generate spurious signals that fall within the FM audio spectrum. Similarly, the imbalance present in the down mixed signal may also distort the FM demodulated audio signal. The resulting audio distortions are often referred to as FM audio artifacts which are considered undesirable from the radio user point of view.
A very low IF receiver, as opposed to a Zero IF receiver, is one in which the received signal is first down-converted to be centered about an IF which is equal to half the channel spacing (i.e. half the bandwidth of the wanted signal), and then it is down-converted again to baseband. While low IF receivers have been able to resolve FM audio artifacts, these receivers still face issues with meeting adjacent channel selectivity, particularly for receivers needing to operate ETSI requirements of 70 dB attenuation at 25 kHz channel spacing.
FIG. 1 is a spectrum graph 100 illustrating the adjacent channel selectivity problem associated with a very low IF receiver. Channel spacing frequency 150 is demarked at 12.5 kHz, spacing increments. The receiver is configured to offset a carrier 102 by minus half the channel spacing (−12.5 kHz) 104 (desired signal at −12.5 KHz) with the presence of both adjacent channel interferers for 25 kHz channel spacing, 106, 108. The image of desired signal 104 is shown at 114 as the image of desired signal at +12.5 KHz, with sideband suppression due to IQ imbalance. The image 118 of the low side adjacent channel interferer 108 falls outside of the filter response 110. However, and undesirably, the image 116 of the high side adjacent channel interferer 106 will become the inband noise and interfere with the desired on-channel signal 104. Furthermore, there will be no protection from analog baseband filters for the high side interferer 106, because the baseband filter response 110 has to extend to accommodate the desired on-channel signal 104. Hence, the adjacent channel selectivity (high side) for these low IF configurations is highly dependent on sideband suppression performance prior to low IF translation.
Accordingly, there is need for an improved receiver to resolve FM audio artifacts.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.
The apparatus and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.